The carbon dioxide (CO.sub.2) laser beam has been used for many years in the ablation of living tissue. The CO.sub.2 laser causes a temperature rise in the tissue primarily due to the absorption of laser radiation by water in the tissue. When this water is heated to its boiling point, it causes an explosive ablation of the surrounding tissue. However, heat transfer to adjacent tissue may cause thermal damage, resulting in tissue necrosis, desiccation, or carbonization ("char") that hinders further ablation until the "charred" tissue is removed. One technique to minimize damaging heat transfer to adjacent, unablated tissue is to cause a rapid temperate rise in irradiated tissue.
One technique that has been developed to cause a rapid temperature rise in irradiated tissue and to minimize thermal damage in adjacent tissue is generically referred to as "superpulse" operation of the CO.sub.2 laser. Superpulse operation involves rapidly heating the irradiated tissue with pulsed laser energy followed by a period of no exposure to laser energy, which gives time for the heat in the non-ablated adjacent tissue to dissipate. The irradiance of the laser beam must be high enough for the absorbed energy to rapidly vaporize water in the target tissue and create an explosive ablation.
In theory, in order to create explosive ablation, tissue irradiance must be above 40 watts/mm.sup.2. However, in practice an irradiance of 70 watts/mm.sup.2 or greater is generally used.
For tissue with a thermal relaxation time of approximately 325 microseconds, the pulse duration of the laser in the superpulse monde is limited to a range of about 150 to 900 microseconds. The "off" time between pulses is typically a minimum of ten time constants or greater than 3.3 milliseconds. While increasing the off time between pulses allows more time for tissue to cool, it has the disadvantage of lowering average power and tissue ablation rates.
The maximum spot size of "char-free" superpulsed ablation is generally limited by the peak power of the laser system being used. The peak power required for superpulse ablation increases by the square of the diameter of the spot. For example, in order to ablate a two-millimeter diameter area, the laser system must be capable of delivering 20 watts of peak power, while a 3 mm spot would require 500 watts peak power. Therefore, to ablate large areas with a laser system having limited peak power, it is necessary to scan the beam over the large area, either by hand or using some type of scanning device. Presently, medical CO.sub.2 laser systems traditionally rely on articulated arms that have the disadvantage of being bulky and using awkward multi-segmented tubes with rotating mirrored joints to deliver the laser energy from the laser console to the treatment site.
Flexible hollow wave guides have been developed that have a more "fiber-like" feel to replace these articulated arms. The disadvantage to such hollow wave guides is they tend to have an energy distribution that is typically non-gaussian or multi-mode and changes as the wave guide is bent. Within a few millimeters of the distal end of the wave guide, the effect of the multi-mode output energy is insignificant. This is because tissue tends to integrate laser energy over small areas and produce fairly uniform "char-free" ablation if the laser is operated in a superpulse mode. However, as the distance between the end of the wave guide and the tissue increases, the output beam diverges to create a large spot size. The increased spot size not only requires increased peak power, but the multi-mode nature of the wave guide output can produce non-uniform ablation. Therefore, it is desirable to maintain a short distance between the end of the wave guide and the target tissue to achieve uniform ablation. This has the disadvantage of limiting the maximum usable spot size even though there is sufficient peak power to ablate larger areas with a single pulse of laser energy.
One solution is to maintain a small spot size and close distance to the target tissue and rapidly move the small beam of laser energy over the target area. A great deal of manual dexterity and experience is required to accomplish uniform ablation over a large area by hand. One proposed mechanical method for doing so is disclosed in U.S. Pat. No. 5,411,502 issued to Zair on May 2, 1995, which is directed to a system using one or two electromechanically rotated mirrors in combination with a focusing lens to cause the laser beam to trace Lissajous figures. The drawbacks to this system are the cumbersome and complex mechanical components and the effort required to maintain the mirrors and focusing lens in precise alignment. In addition, this proposed system does not address compatibility with pulsed laser beam radiation or flexible hollow wave guide systems.
Consequently, there is a need for a mechanically simple system for uniformly and thoroughly scanning a large target area with a beam of laser energy.